6 Random access procedure
25.2143GPPPhysical layer procedures (FDD)Release 17TS
6.1 Physical random access procedure
The physical random access procedure described in this subclause is initiated upon request from the MAC sublayer (cf. [9]). This procedure can also be initiated if indicated by higher layers and Fallback_R99_Enabled is TRUE.
Before the physical random-access procedure can be initiated, Layer 1 shall receive the following information from the higher layers (RRC):
– The preamble scrambling code.
– The message length in time, either 10 or 20 ms.
– The AICH_Transmission_Timing parameter [0 or 1].
– The set of available signatures and the set of available RACH sub-channels for each Access Service Class (ASC). Sub-channels are defined in subclause 6.1.1.
– The power-ramping factor Power Ramp Step [integer > 0].
– The parameter Preamble Retrans Max [integer > 0].
– The initial preamble power Preamble_Initial_Power.
– The Power offset P p-m = Pmessage-control – Ppreamble, measured in dB, between the power of the last transmitted preamble and the control part of the random-access message.
– The set of Transport Format parameters. This includes the power offset between the data part and the control part of the random-access message for each Transport Format.
Note that the above parameters may be updated from higher layers before each physical random access procedure is initiated.
At each initiation of the physical random access procedure, Layer 1 shall receive the following information from the higher layers (MAC):
– The Transport Format to be used for the PRACH message part.
– The ASC of the PRACH transmission.
– The data to be transmitted (Transport Block Set).
The physical random-access procedure shall be performed as follows:
1 Derive the available uplink access slots, in the next full access slot set or in the next 8 access slots, for the set of available RACH sub-channels within the given ASC with the help of subclauses 6.1.1. and 6.1.2. Randomly select one access slot among the ones previously determined. If there is no access slot available in the selected set, randomly select one uplink access slot corresponding to the set of available RACH sub-channels within the given ASC from the next access slot set or in the next 7 access slots. The random function shall be such that each of the allowed selections is chosen with equal probability.
2 Randomly select a signature from the set of available signatures within the given ASC. The random function shall be such that each of the allowed selections is chosen with equal probability.
3 Set the Preamble Retransmission Counter to Preamble Retrans Max.
4 If the Preamble_Initial_Power is below the minimum level required in [7], set the Commanded Preamble Power to a value, which shall be at or above the Preamble_Initial_Power and at or below the required minimum power specified in [7]. Otherwise set the parameter Commanded Preamble Power to Preamble_Initial_Power.
5 In the case that the Commanded Preamble Power exceeds the maximum allowed value, set the preamble transmission power to the maximum allowed power. In the case that the Commanded Preamble Power is below the minimum level required in [7], set the preamble transmission power to a value, which shall be at or above the Commanded Preamble Power and at or below the required minimum power specified in [7]. Otherwise set the preamble transmission power to the Commanded Preamble Power. Transmit a preamble using the selected uplink access slot, signature, and preamble transmission power.
6 If no positive or negative acquisition indicator (AI +1 nor –1) corresponding to the selected signature is detected in the downlink access slot corresponding to the selected uplink access slot:
6.1 Select the next available access slot in the set of available RACH sub-channels within the given ASC.
6.2 Randomly select a new signature from the set of available signatures within the given ASC. The random function shall be such that each of the allowed selections is chosen with equal probability.
6.3 Increase the Commanded Preamble Power by P0 = Power Ramp Step [dB]. If the Commanded Preamble Power exceeds the maximum allowed power by 6dB, the UE may pass L1 status ("No ack on AICH") to the higher layers (MAC) and exit the physical random access procedure.
6.4 Decrease the Preamble Retransmission Counter by one.
6.5 If the Preamble Retransmission Counter > 0 then repeat from step 5. Otherwise pass L1 status ("No ack on AICH") to the higher layers (MAC) and exit the physical random access procedure.
7 If a negative acquisition indicator corresponding to the selected signature is detected in the downlink access slot corresponding to the selected uplink access slot, pass L1 status ("Nack on AICH received") to the higher layers (MAC) and exit the physical random access procedure.
8 Transmit the random access message three or four uplink access slots after the uplink access slot of the last transmitted preamble depending on the AICH transmission timing parameter. Transmission power of the control part of the random access message should be P p-m [dB] higher than the power of the last transmitted preamble. Transmission power of the data part of the random access message is set according to subclause 5.1.1.2.
9 Pass L1 status "RACH message transmitted" to the higher layers and exit the physical random access procedure.
6.1.1 RACH sub-channels
A RACH sub-channel defines a sub-set of the total set of uplink access slots. There are a total of 12 RACH sub-channels. RACH sub-channel #i (i = 0, …, 11) consists of the following uplink access slots:
– Uplink access slot #i leading by p-a chips the downlink access slot #i contained within the 10 ms interval that is time aligned with P-CCPCH frames for which SFN mod 8 = 0 or SFN mod 8 = 1.
– Every 12th access slot relative to this access slot.
The access slots of different RACH sub-channels are also illustrated in Table 7.
Table 7: The available uplink access slots for different RACH sub-channels
|
SFN modulo 8 of corresponding P-CCPCH frame |
Sub-channel number |
|||||||||||
|
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
|
|
0 |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
||||
|
1 |
12 |
13 |
14 |
8 |
9 |
10 |
11 |
|||||
|
2 |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
||||
|
3 |
9 |
10 |
11 |
12 |
13 |
14 |
8 |
|||||
|
4 |
6 |
7 |
0 |
1 |
2 |
3 |
4 |
5 |
||||
|
5 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|||||
|
6 |
3 |
4 |
5 |
6 |
7 |
0 |
1 |
2 |
||||
|
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
|||||
6.1.2 RACH access slot sets
The PRACH contains two sets of access slots as shown in Figure 2. Access slot set 1 contains PRACH slots 0 – 7 and starts p-a chips before the downlink P-CCPCH frame for which SFN mod 2 = 0. Access slot set 2 contains PRACH slots 8 – 14 and starts (p-a –2560) chips before the downlink P-CCPCH frame for which SFN mod 2 = 1.
Figure 2: PRACH access slot and downlink AICH relation (p-a = 7680 chips)
6.1A Physical random access procedure for Enhanced Uplink in CELL_FACH state and IDLE mode
The physical random access procedure described in this subclause is initiated upon request from the MAC sublayer (cf. [9]).
Before the physical random-access procedure can be initiated, Layer 1 shall receive the following information from the higher layers (RRC):
– Preamble scrambling code(s) for 2 and 10ms TTI.
– The AICH_Transmission_Timing parameter [0 or 1].
– The set of available signatures and the set of available RACH sub-channels corresponding to E-DCH resources for each Access Service Class (ASC). Sub-channels are defined in subclause 6.1.1.
– The total number of E-DCH resources configured in the cell.
– The power-ramping factor Power Ramp Step [integer > 0].
– The parameter Preamble Retrans Max [integer > 0].
– The initial preamble power Preamble_Initial_Power.
– The Power offset Pp-e = Pdpcch – Ppreamble, measured in dB, between the power of the last transmitted preamble and the initial DPCCH transmission power.
– The number of TTIs in which only the uplink DPCCH is sent before the E-DCH transmission may start for 2 and 10ms TTI.
Note that the above parameters may be updated from higher layers before each physical random access procedure is initiated.
At each initiation of the physical random access procedure, Layer 1 shall receive the following information from the higher layers (MAC):
– The ASC of the PRACH transmission.
If Concurrent_TTI_Deployment_Enabled is TRUE, Layer 1 shall also receive the following additional parameters from the higher layers (MAC):
– TTI length, either 2 or 10ms.
The procedure for the selection of the TTI length is defined in subclause 8.5.b1 in [5]. The TTI length may be updated by higher layers before each preamble retransmission.
The physical random-access procedure shall be performed as follows:
1 If NT-HS-DPCCH_Enabled is FALSE, or if NT-HS-DPCCH_Enabled is TRUE and the physical random-access procedure is not triggered by an HS-SCCH order, then derive the available uplink access slots, in the next full access slot set or in the next 8 access slots, for the set of available RACH sub-channels within the given ASC with the help of subclauses 6.1.1. and 6.1.2. Randomly select one access slot among the ones previously determined. If there is no access slot available in the selected set, randomly select one uplink access slot corresponding to the set of available RACH sub-channels within the given ASC from the next access slot set or in the next 7 access slots. The random function shall be such that each of the allowed selections is chosen with equal probability. If NT-HS-DPCCH_Enabled is TRUE and the physical random-access procedure is triggered by an HS-SCCH order, then the next available access slot is chosen.
2 If Concurrent_TTI_Deployment_Enabled is TRUE, the scrambling code corresponding to the indicated TTI length is chosen.
2a Randomly select a signature from the set of available signatures within the given ASC and indicated scrambling code. The random function shall be such that each of the allowed selections is chosen with equal probability. If Concurrent_TTI_Deployment_Enabled is TRUE, the set of available signatures corresponds to the indicated TTI length.
3 Set the Preamble Retransmission Counter to Preamble Retrans Max.
4 If the Preamble_Initial_Power is below the minimum level required in [7], set the Commanded Preamble Power to a value, which shall be at or above the Preamble_Initial_Power and at or below the required minimum power specified in [7]. Otherwise set the parameter Commanded Preamble Power to Preamble_Initial_Power.
5 In the case that the Commanded Preamble Power exceeds the maximum allowed value, set the preamble transmission power to the maximum allowed power. In the case that the Commanded Preamble Power is below the minimum level required in [7], set the preamble transmission power to a value, which shall be at or above the Commanded Preamble Power and at or below the required minimum power specified in [7]. Otherwise set the preamble transmission power to the Commanded Preamble Power. Transmit a preamble using the selected uplink access slot, signature, and preamble transmission power.
6 If no positive or negative acquisition indicator (AI +1 nor –1) corresponding to the selected signature is detected in the downlink access slot corresponding to the selected uplink access slot:
6.1 Select the next available access slot in the set of available RACH sub-channels within the given ASC.
6.2 Randomly select a new signature from the set of available signatures within the given ASC. The random function shall be such that each of the allowed selections is chosen with equal probability. If an update to the TTI length is indicated by higher layers, then the set of available signatures and the preamble scrambling code are also updated correspondingly.
6.3 Increase the Commanded Preamble Power by P0 = Power Ramp Step [dB]. If the Commanded Preamble Power exceeds the maximum allowed power by 6dB, the UE may pass L1 status ("No ack on AICH") to the higher layers (MAC) and exit the physical random access procedure.
6.4 Decrease the Preamble Retransmission Counter by one.
6.5 If the Preamble Retransmission Counter > 0 then repeat from step 5. Otherwise pass L1 status ("No ack on AICH") to the higher layers (MAC) and exit the physical random access procedure.
7 If a negative acquisition indicator on AICH corresponding to the selected signature is detected in the downlink access slot corresponding to the selected uplink access slot.
7.1 If no Extended Acquisition Indicator signature set is configured in the cell, pass L1 status (“Nack on AICH received”) to the higher layers (MAC) and exit the physical random access procedure.
7.2 If an Extended Acquisition Indicator signature set is configured in the cell, detect which one of the defined Extended Acquisition Indicator signatures is present.
7.2.1 If the detected Extended Acquisition Indicator signature and modulation symbol corresponds to ‘NACK’ as defined in [1], pass L1 status (“Nack on AICH received”) to the higher layers (MAC) and exit the physical random access procedure.
7.2.2 If the detected Extended Acquisition Indicator signature and modulation symbol do not correspond to ‘NACK’, pass L1 status (“Ack on AICH received”) with the corresponding E-DCH resource index as defined in [1] to higher layers (MAC) and skip step 8.
8 If a positive acquisition indicator on AICH corresponding to the selected signature is detected in the downlink access slot corresponding to the selected uplink access slot, pass L1 status (“Ack on AICH received”) with the default E-DCH resource index corresponding to the selected signature as defined in [1] to higher layers (MAC),
9 Start transmitting DPCCH (timing as specified in [1] and synchronization according to synchronization procedure AA). The initial transmission power of DPCCH prior to starting the E-DCH transmission should be
Pp-e [dB] higher than the power of the last transmitted preamble.
10 Proceed to transmitting E-DPCCH and E-DPDCH, and transmit HS-DPCCH if so instructed by MAC layer after the defined number of TTIs of DPCCH only transmission has passed.